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Why is this done by the users? For what purpose? Doesn't it slow down the system?
example:jfs for / ext2 for /boot reiser4 for /var so you see this there will be an interaction between different file systems and i think this can slow down the system.
Each filesystem has its strengths. For example, ext4 is simple and functional, btrfs is specialized for data storage (at least when it is end user ready), f2fs is optimized for flash memory storage and reiserfs is good at handling millions of small files. Hence, a user may want to format its system partition to ext4, bulk data partition to btrfs, flash drive or SSD to f2fs and database with many files to reiserfs. Sometimes if a user dual-boots with Windows, the system will also have an NTFS, not to mention BIOS and EFI bootloader partitions are AFAIK always in fat32.
Usually the intention is to optimize performance by chosing a filesystem which better suits the purpose and type of files stored beneath that path. For example, some filesystems handle many small files better than others.
Take a look at https://en.wikipedia.org/wiki/Comparison_of_file_systems to get an idea.
